1. Field of the Invention
This invention relates to an apparatus for adjusting a position of a rotary head in a magnetic recording/reproducing apparatus such as a video cassette recorder (VCR).
As shown in FIGS. 1A and 1B, recent high quality consumer VCRs have several rotary heads 1 on a rotary drum 2 for recording video signals, high fidelity audio signals and for erasing recorded tracks. All heads are bonded on small metal plates 3a, called a head base, and the rotary drum 2 is mounted on a fixed drum 4.
All heads should be precisely positioned on a rotary drum 2 in order to satisfy the desired recording format on the tape, because the arrangement of recorded tracks are mainly determined by the head positions on the rotary drum. The head position includes absolute head height Ha which is the distance between a datum plane 5 of the fixed drum 4 and a track edge 6 of the head 1, relative head height Hr which is the relative head height among the track edges of different heads, and gap azimuth angle 31 of the head 1.
The conventional manner of adjusting the head position is as follows:
Consumer VTRs are not provided with a mechanism for adjusting the azimuth angle, and the angle is adjusted in the manufacturing process of the head when cutting out the chip from the gapped bar. Therefore, the precision of azimuth angle depends on the machining precision and the precision of adhering the head 1 to the head base 3a with an adhesive 8.
On the other hand, to adjust the head height, first the height H0 of each head from the surface (the head adhesion surface) of the head base 3a of the track edge 6 is measured, and the absolute height Ha of each head is roughly adjusted by placing a metal plate 9 called a shim having a thickness corresponding to a required adjusting amount on the basis of the measured H0 value between the head base 3a and the rotary drum 2, and each head is fixed to the rotary drum with screws.
Consequently, the rotary drum 2 on which the heads are mounted is fixed on the fixed drum 4, and the relative height of the heads is precisely adjusted using a screw 10 of which a tip is contacting with a part of the head base as shown in FIG. 1b. More specifically, by tightening or loosening the screw 10, the head base is elastically deformed, and the head chip adhered to the tip of the head base is moved vertically in the head height direction (the direction parallel to the rotary shaft A off the rotary drum) as indicated by arrow B in the drawing, and Ha and Hr are adjusted precisely. However, since the fluctuations of the tip shape of the screw 10 are significant and the height is changed as much as 500 microns by turning the screw 10 by a revolution, the precise adjustment of the height in the sub-micron order is not realized.
The basis of the above conventional technique is the mechanical deformation of the head base by pushing a small screw within its elastic limits. Accordingly, when the drum unit is subjected to vibratory or temperature changing circumstances, the small screw pushing the head base may be loosened and the head height may be fluctuated.
In our experiments, in an ambient temperature cycle test of -40 degrees and +80 degrees and a vibration test, it was found that the fluctuations of the relative head height were about 2 microns.
The influence of the above fluctuations on the reproduced picture qualities is very severe, especially in the long play mode, for example, the six-hour mode in the VHS system of narrow track recording, because the above fluctuations will cause tracking errors and decrease of reproducing voltage and increase of cross talk interference from adjacent tracks.
Furthermore, if a fluid bearing is adopted as the bearing of the rotary head drum, the yawing of the rotary head drum during rotating depends on the rotation of the rotary drum. It indicates that the absolute head height, relative head height and azimuth angle also depend on the rotation of the rotary head drum. Consequently, by the conventional head adjusting method, in which the head position should be adjusted without rotating the drum, it is very difficult to guarantee a high adjustment accuracy.